[Vm-dev] Slang newbie question

Ronald Spengler ron.spengler at gmail.com
Tue Sep 15 04:52:06 UTC 2009

Yikes. Looks like returning a string from the prim isn't easy either.
I googled and didn't find anything in particular; is there a generally
accepted approach to doing this?

I already have:

	self returnTypeC: 'char *'.

The best I could find was TR suggesting that one find a primitive that
already knew what to do with a char* (I grok that the pointer is
unsafe if garbage collection has occured:)


On Mon, Sep 14, 2009 at 8:30 PM, Andreas Raab <andreas.raab at gmx.de> wrote:
> Ronald Spengler wrote:
>> Okay, reread the code, realized that the comments actually answer
>> every question I have except "how do I guarantee that the garbage
>> collector doesn't run"
> Don't call functions that allocate objects. A GC may happen if you call a
> function that allocates an object, including, but not limited to,
> instantiateClassIndexableSize(); makePointwithXValueyValue() and others.
> This will hopefully change in the future to a scheme where primitives
> *never* cause GC unless explicitly requested (and rather fail the allocation
> and have the plugin deal with that failure) but for now, the only thing you
> can do is to either avoid allocations altogether (which isn't as bad as it
> sounds since mostly you don't need to allocate Squeak objects from
> primitives) or do it at the end of the primitive where you can ignore the
> inputs (since they have been used already) and just construct a result, or
> do proper remapping of your inputs (listed in increasing number of
> difficulty).
> If someone has a list of interpreter proxy functions handy, we can tell you
> which ones may cause GC and which ones are GC-safe.
> Cheers,
>  - Andreas
>> On Mon, Sep 14, 2009 at 7:36 PM, Ronald Spengler <ron.spengler at gmail.com>
>> wrote:
>>> Thanks Dave! That helps a lot. This snippet scares me a little bit
>>> though:
>>> "in a section of code in which the garbage collector is guaranteed not to
>>> run"
>>> I realize now that:
>>>  - I don't know how to guarantee that the gc won't run
>>>  - My C library will take it's sweet time running, and it's runtime is
>>> a function of it's input, could be forever in the extreme case.
>>> So, to be safe, on the C side of things, should I copy the string
>>> ASAP? Or does C code escape the garbage collector? Is it safe to
>>> malloc()?
>>> Thanks again for your help, and please forgive my ignorance.
>>> On Sun, Sep 13, 2009 at 6:56 PM, David T. Lewis <lewis at mail.msen.com>
>>> wrote:
>>>> On Sun, Sep 13, 2009 at 01:23:32PM -0700, Ronald Spengler wrote:
>>>>> Hello everyone.
>>>>> I have a named primitive, and I need to send a ByteString to it, to be
>>>>> processed and returned by an external library. To get a string into
>>>>> Slang,
>>>>> should I send it #asByteArray, and would that let me treat the bytes as
>>>>> integers on the stack? I'm basically trying to get a char* on the other
>>>>> side.
>>>> You can use the ByteString as a parameter to the primitive, no problem.
>>>> The only tricky bit is that C expects null terminated strings, so you
>>>> need
>>>> to copy the contents of the ByteString into a null terminated array
>>>> before
>>>> you can let it be used by the C library as a char *.
>>>> I'm sure there are lots of examples, but you can look at
>>>> OSProcessPlugin>>cStringFromString: and
>>>> OSProcess>>transientCStringFromString:
>>>> for examples of how to copy the string buffer into a null terminated
>>>> buffer
>>>> for use in C. Look at senders of these two methods for examples of
>>>> primitives
>>>> that pass strings as parameters. (OSProcessPlugin is on SqueakSource if
>>>> you
>>>> do not have it).
>>>> Following are a couple of examples taken from OSPP. In both cases, a
>>>> buffer
>>>> is allocated with size one greater than the string length, and the
>>>> contents
>>>> of the Smalltalk string are copied into the buffer space with a trailing
>>>> null terminator. The #primitiveChdir example allocates a new Smalltalk
>>>> string to use for the buffer, and #primitivePutEnv uses malloc to
>>>> allocate
>>>> the new buffer (because in this case the buffer must be "permanently"
>>>> valid
>>>> after the primitive exits).
>>>> primitiveChdir
>>>>       "Call chdir(2) to change current working directory to the
>>>> specified path string. Answer
>>>>       nil for success, or errno on failure."
>>>>       | path errno |
>>>>       self export: true.
>>>>       self var: 'path' type: 'char *'.
>>>>       self var: 'errno' type: 'extern int'.
>>>>       path := self transientCStringFromString: (interpreterProxy
>>>> stackObjectValue: 0).
>>>>       (self chdir: path)
>>>>               ifTrue: [interpreterProxy pop: 2; push: interpreterProxy
>>>> nilObject]
>>>>               ifFalse: [interpreterProxy pop: 2; pushInteger: errno].
>>>> transientCStringFromString: aString
>>>>       "Answer a new null-terminated C string copied from aString.
>>>>       The string is allocated in object memory, and will be moved
>>>>       without warning by the garbage collector. Any C pointer
>>>>       reference the the result is valid only until the garbage
>>>>       collector next runs. Therefore, this method should only be used
>>>>       within a single primitive in a section of code in which the
>>>>       garbage collector is guaranteed not to run. Note also that
>>>>       this method may itself invoke the garbage collector prior
>>>>       to allocating the new C string.
>>>>       Warning: The result of this method will be invalidated by the
>>>>       next garbage collection, including a GC triggered by creation
>>>>       of a new object within a primitive. Do not call this method
>>>>       twice to obtain two string pointers."
>>>>       | len stringPtr newString cString |
>>>>       self returnTypeC: 'char *'.
>>>>       self var: 'stringPtr' declareC: 'char *stringPtr'.
>>>>       self var: 'cString' declareC: 'char *cString'.
>>>>       len := interpreterProxy sizeOfSTArrayFromCPrimitive:
>>>> (interpreterProxy arrayValueOf: aString).
>>>>       "Allocate space for a null terminated C string."
>>>>       interpreterProxy pushRemappableOop: aString.
>>>>       newString := interpreterProxy
>>>>               instantiateClass: interpreterProxy classString
>>>>               indexableSize: len + 1.
>>>>       stringPtr := interpreterProxy arrayValueOf: interpreterProxy
>>>> popRemappableOop.
>>>>       cString := interpreterProxy arrayValueOf: newString.
>>>>  "Point to the actual C string."
>>>>       self cCode: '(char *)strncpy(cString, stringPtr, len)'.
>>>> "Make a copy of the string."
>>>>       cString at: (len) put: 0.
>>>> "Null terminate the C string."
>>>>       ^ cString
>>>> primitivePutEnv
>>>>       "Set an environment variable using a string of the form
>>>> 'KEY=value'. This
>>>>       implementation allocates a C string using malloc to allocate from
>>>> the C heap
>>>>       (using cStringFromString rather than transientCStringFromString).
>>>> This
>>>>       is necessary because the C runtime library does not make a copy of
>>>> the
>>>>       string into separately allocated environment memory."
>>>>       | cStringPtr keyValueString |
>>>>       self export: true.
>>>>       self var: 'cStringPtr' declareC: 'char *cStringPtr'.
>>>>       keyValueString := interpreterProxy stackObjectValue: 0.
>>>>       cStringPtr := self cStringFromString: keyValueString.
>>>>       ((self putenv: cStringPtr) == 0)        "Set environment
>>>> variable."
>>>>               ifTrue: [interpreterProxy pop: 2; push: keyValueString]
>>>>               ifFalse: [^ interpreterProxy primitiveFail]
>>>> cStringFromString: aString
>>>>       "Answer a new null-terminated C string copied from aString. The C
>>>> string
>>>>       is allocated from the C runtime heap. See
>>>> transientCStringFromString for
>>>>       a version which allocates from object memory.
>>>>       Caution: This may invoke the garbage collector."
>>>>       | len sPtr cString |
>>>>       self returnTypeC: 'char *'.
>>>>       self var: 'sPtr' declareC: 'char *sPtr'.
>>>>       self var: 'cString' declareC: 'char *cString'.
>>>>       sPtr := interpreterProxy arrayValueOf: aString.
>>>>       len := interpreterProxy sizeOfSTArrayFromCPrimitive: sPtr.
>>>>       cString := self callocWrapper: len + 1 size: 1.         "Space for
>>>> a null terminated C string."
>>>>       self cCode: '(char *) strncpy (cString, sPtr, len)'.    "Copy the
>>>> string."
>>>>       ^ cString
>>>> callocWrapper: count size: objectSize
>>>>       "Using malloc() and calloc() is something I would like to avoid,
>>>> since it is
>>>>       likely to cause problems some time in the future if somebody
>>>> redesigns
>>>>       object memory allocation. This wrapper just makes it easy to find
>>>> senders
>>>>       of calloc() in my code. -dtl"
>>>>       self returnTypeC: 'void *'.
>>>>       ^ self cCode: 'calloc(count, objectSize)'
>>>> Dave
>>> --
>>> Ron


More information about the Vm-dev mailing list